1. Field of the Invention
This invention relates to metalorganic chemical vapor deposition (MOCVD) of strontium bismuth tantalate, and in particular to a toluene-based solvent system for precursor delivery for MOCVD of such material
2. Description of the Related Art
Liquid delivery MOCVD is the preferred method for depositing thin films of ferroelectric materials because of the ability to carefully control composition and deposit conformal films of high density. In such a process a solution or suspension of precursors (metalorganic source reagents for the respective metal components of the product film material) is vaporized, preferably at high rate by xe2x80x9cflash vaporizationxe2x80x9d techniques to produce a corresponding precursor vapor which may be mixed with carrier and/or additive gases (e.g., oxygen-containing gas, inert gases such as argon, helium, etc., co-reactive gases, diluents, etc.) to form a vapor mixture. The vapor mixture then is flowed to a deposition zone where the precursor mixture is contacted with a substrate at elevated temperature to effect deposition from the vapor phase onto the substrate of a desired material.
MOCVD of SrBi2Ta2O9 (SBT) and related materials use precursors that are chemically compatible in solution for long periods of time and also have similar decomposition characteristics vis-à-vis one another in the MOCVD process. Sr(thd)2-LBA, Bi(thd)3, and Ta(O-i-Pr)4(thd) (where thd stands for 2,2,7,7-tetramethyl-3,5-heptanedionato, LBA stands for a Lewis base adduct and O-i-Pr stands for isopropoxide) provide a preferred combination of precursors for such purpose. Precursors for deposition of SBT and other ferroelectric materials, and their use in liquid delivery MOCVD formation of high quality product films, are more fully described in U.S. patent application Ser. No. 08/960,915 filed Oct. 30, 1997 in the names of Thomas H. Baum, et al., now issued as U.S. Pat. No. 5,859,274, and U.S. patent application Ser. No. 08/976,087 filed Nov. 20, 1997 in the names of Frank S. Hintermaier, et al.
The best previously known solvent system for this precursor suite (of Sr(thd)2-LBA, Bi(thd)3, and Ta(O-i-Pr)4(thd)) consisted of alkanes and in particular a mixture of octane, decane, with an excess of the LBA of the strontium precursor, Sr(thd)2. In particular, a ratio of 5 parts octane, 4 parts decane, and 1 part LBA was found to maintain the precursors in a stable condition and deliver them reliably to a flash vaporization equipment. Solvent compositions of such type are more fully described in U.S. patent application Ser. No. 08/975,372 filed Nov. 20, 1997 in the names of Thomas H. Baum, et al., now issued as U.S. Pat. No. 5,916,359.
However, the alkane-based solvents have limited solubility of the aforementioned SBT precursors, especially for the Sr(thd)2, which only dissolves to about 0.2 M in a 5:4:1 mixture of octane:decane:pmdeta. This limited solubility characteristic of the solvent composition has disadvantages for liquid delivery vaporization and the subsequent deposition process. During vaporization, wherein the precursor solution typically is flowed through a liquid delivery tube to a heated surface, the molarity of the precursor solution must remain significantly below the solubility limit to prevent precipitation of solid metalorganic particles from the solution, which would otherwise occur as solvent boils off in the liquid delivery tube, and which would lead to clogging of the liquid delivery tube. In addition, low solution molarity requires the vaporization of more solvent in order to deliver the same amount of precursor to the process. This creates undue requirements for the supply of heat to the metalorganics in the vaporizer, since more heat must then be provided to volatilize the additional solvent that is present.
The deposition process itself also is adversely affected by low molarity solutions. By way of example, film growth rates were limited to 3-4 nanometers per minute (nm/min) in a prototype MOCVD reactor delivering a 0.30M solution at a rate of 0.2 ml/min. Growth rates should be 2-3 times higher than this level in order to meet the requirements of a xe2x80x9cmanufacturable process.xe2x80x9d In addition, limiting the rate of delivery of precursor to the substrate surface makes it more difficult to push the process into a surface kinetic-controlled growth regime where conformality to non-planar structures is improved, relative to film formation processes operating outside of such regime.
It would therefore be an advance in the art to provide a solvent medium for liquid delivery MOCVD of precursor compositions such as SBT, which overcome the aforementioned difficulties.
The present invention relates to a solvent composition that is usefully employed for liquid delivery MOCVD applications, e.g., as a solvent medium for SBT, doped SBT and other ferroelectric precursors.
In one aspect, the invention relates to a solvent composition useful for liquid delivery MOCVD, comprising toluene and a Lewis base, wherein toluene is present at a concentration of from about 75% to about 98% by volume, based on the total volume of toluene and the Lewis base.
Another aspect of the invention relates to a precursor composition useful for liquid delivery MOCVD, comprising (i) a solvent composition including toluene and a Lewis base, wherein toluene is present at a concentration of from about 75% to about 98% by volume, based on the total volume of toluene and the Lewis base, and (ii) a precursor species dissolved or suspended in the solvent composition.
A still further aspect of the invention relates to a method of forming a material film on a substrate, comprising:
providing a precursor composition for at least one component of the material film; volatilizing the precursor composition to yield a precursor vapor; and contacting the precursor vapor with the substrate to deposit said at least one component of the material film thereon;
wherein the precursor composition comprises (i) a solvent composition including toluene and a Lewis base, wherein toluene is present at a concentration of from about 75% to about 98% by volume, based on the total volume of toluene and the Lewis base, and (ii) a precursor species dissolved or suspended in the solvent composition.
Other objects, features and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.